Fluid flow control system employing a plurality of digital valve elements

ABSTRACT

An upstream fluid chamber adapted for fluid flow in a given direction is interconnected to a downstream fluid chamber by a plurality of individually actuatable digital valve elements disposed in a plane substantially transverse to the given direction so that the pressure drop across each valve element is substantially the same. The valve elements are disposed so the fluid from the upstream chamber passing through the valve elements converges in the downstream chamber to dissipate the vena contracta. The areas of the valve elements are weighted so the smaller areas follow a geometric progression while the larger areas deviate from a geometric progression. Preferably, at least the two larger areas are the same size. The valve elements themselves each have an orifice, a plug, and means for maintaining the plug in one of two positions. The plug seals the orifice in the one positon and lies outside of the fluid stream in the static fluid region in the other position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of a copending application, Ser. No. 703,468, nowabandoned.

BACKGROUND OF THE INVENTION

This invention relates to the control of fluid flow and, moreparticularly, to a digital fluid flow control system.

Conventionally, a single element analog valve is employed to control therate of flow in a fluid system. The essential components of such a valveare an orifice through which the fluid flows and a plug that is movableinto and out of the orifice. The flow rate through the valve isdetermined by the extent that the plug blocks the orifice. There arealso fluid control systems in the prior art that utilize multiple valveelements. They are, for the most part, digital flow control devicescomprising a plurality of digital valve elements capable of assuming oneof two stable states--open or closed. Generally, the orifice areas areweighted to follow a geometric progression of two.

Schmohl et al. U.S. Pat. No. 2,229,903, issued Jan. 28, 1941, disclosesa digital valve in which two parallel laterally displaced conduits areconnected by a plurality of cross ports distributed at different pointsalong the length of the conduits. These cross ports have orifices withdifferent areas and individually actuatable plugs blocking the orifices.It is taught that the orifices are selectively unblocked eitherindividually or in combinations to control the rate of flow through thevalve. In this way, many more different flow rates can be established bythe valve than there are individual orifices.

Dufour U.S. Pat. No. 3,063,468, issued Nov. 13, 1962, discloses aplurality of valve elements stacked adjacent to one another along thedirection of fluid flow. The valve elements take the form of discs thatare individually rotatable into either of two positions. Apertures arearranged on the discs so that a different number of apertures of thediscs are aligned for each combination of disc positions. The number ofaligned apertures follows a geometric progression of two as successivediscs are rotated. Fluid flows through the aligned apertures. Thus, themore apertures that are aligned, the more fluid flows through thesystem.

U.S. Pat. No. 3,072,146, issued Jan. 8, 1963, to T. Gizeski, is directedto a digital regulator valve in which transverse inlet and outletmanifolds are linked at different points along their lengths by conduitshaving digital control valves with orifice areas following a geometricprogression of two. An upstream conduit feeds the inlet manifold, andthe outlet manifold supplies a downstream conduit. A digital programmercontrols the operation of the digital valve elements and accordingly therate of fluid flow through the system.

Ernyei U.S. Pat. No. 3,331,393, issued July 18, 1967, discloses a fluidflow control system employing balanced digital valve elements. Theupstream conduit of the fluid system is connected to a first disc-shapedcavity and the downstream conduit of the fluid system is connected tosecond and third disc-shaped cavities located on either side of thefirst cavity. Each digital valve element cuts through the three cavitiesat a different point, having an orifice between the first cavity and thesecond cavity and an orifice between the first cavity and the thirdcavity. Two plugs mounted on the same rod control the fluid flow throughthe orifices of each valve element. The force exerted on one of theplugs due to the pressure drop across its orifice is balanced by theforce exerted on the other plug due to the pressure drop across itsorifice.

The prior art fluid flow control systems employing multiple digitalvalve elements suffer numerous shortcomings that are especially seriousat high pressures and fluid flow rates. First, the valve elements aredistributed along the direction of fluid flow in the system. As aresult, the pressure drops across the valve elements differ from oneanother and are dependent on the states of the other valve elements.Correspondingly, the flow rates through the valve elements are alsointerdependent, i.e., not solely a function of the orifice areas. In adigital fluid flow control system this interdependence is manifested asa deviation in the flow rate from the nominal digital values.

Second, the problem of the formation of a vena contracta by the fluidpassing through the digital valve elements is not met. At high flowrates, the vena contracta causes the effective orifice area of the valveelements to become pressure dependent, thereby introducing a source ofunlinearity into the fluid control system. In addition, the venacontracta frequently triggers cavitation, which causes pitting of thevalve parts and inefficient operation.

Third, the valve elements of a digital fluid flow control systeminvariably have orifice areas that are weighted to follow a geometricprogression of two. Accordingly, one-half of the maximum flow ratethrough the fluid flow control system is attributable to only one valveelement. Full advantage is not then taken of the potentialsimplifications in valve design made possible by the fact the fluid ishandled by a plurality of valve elements rather than a single element inan analog valve. Further, each time the most significant valve elementis opened or closed, an extraordinary disturbance may be created in thefluid system because of the possible differences in opening and closingtime of the different elements

Fourth, it is difficult to design the digital valve elements themselvesso the fluid rate in the open state remains constant in the course ofprolonged use. Each valve element comprises a flow determining orificeand a plug that assumes one of two stable positions. When the valveelement is open, the plug is positioned to seal the orifice. When thevalve element is closed, the plug is positioned in the fluid stream inspaced relationship from the orifice so the orifice is unblocked. In theopen state of the valve element, the extent to which the orifice isunblocked depends upon the position of the plug since the plug is in thefluid stream. As the moving parts of the valve element wear withprolonged use, the position of the plug in the fluid stream in the openstate varies, and the flow rate varies accordingly. In other words, theplug tends to modulate the flow rate in the same manner as an analogvalve.

SUMMARY OF THE INVENTION

In one aspect, the invention contemplates the disposition of a pluralityof individually actuatable digital valve elements in a digital fluidflow control system so the fluid from the upstream chamber passingthrough the open valve elements converges in the downstream chamber todissipate the vena-contracta. Therefore, high pressures and flow ratescan be accommodated without causing cavitation and the effective orificearea of the open valve elements is independent of the fluid pressure.

In another aspect, the invention contemplates a digital fluid flowcontrol system in which the areas of the digital valve elements areweighted so the smaller areas follow a geometric progression and thelarger areas deviate from a geometric progression. Preferably, two ormore of the largest areas are the same size. Thus, no single valveelement is responsible for as much as one-half of the maximum fluid flowrate of the system. Smoother operation and faster response of the fluidflow control system are possible.

A feature of the invention involves the disposition of a plurality ofindividually actuatable digital valve elements in a plane substantiallytransverse to the direction of flow in a digital fluid flow controlsystem so the pressure drop across each valve element is substantiallythe same. Consequently, the rate of fluid flow through each open valveelement is independent of the state, open or closed, of the other valveelements. Preferably, the plurality of valve elements are adapted forconverging radial fluid flow from an upstream conduit of large diameterto a downstream conduit of smaller diameter concentric with the upstreamconduit.

Another feature of the invention involves the digital valve elementsthemselves. The movement of a plug of a digital valve element iscontrolled so it assumes one of two stable positions. In the oneposition the plug lies outside of the fluid stream in the static fluidregion and the valve element is open. In the other stable position theplug seals an orifice and the valve element is closed. As a result,small changes in the position of the plugs in a digital fluid controlsystem that develop in the course of prolonged use do not affect thefluid flow rate through the open valve elements, i.e., the plugs do notmodulate the flow rate.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of specific embodiments of the invention are illustrated inthe drawings, in which:

FIGS. 1A and 1B are a front elevation view in section and a sideelevation view in section, respectively, of a fluid flow control systemincorporating the principles of the invention;

FIG. 2 is a perspective view partially in section of another embodimentof a fluid flow control system incorporating the principles of theinvention;

FIG. 3 is a perspective view partially in section of a modification ofthe embodiment of FIG. 2;

FIG. 4 is a side elevation view in section of another embodiment of afluid control system incorporating the principles of the invention;

FIGS. 5A and 5B are a side elevation view in section and a partialenlargement, respectively, of one of the valve elements employed in thefluid control system of FIG. 1;

FIG. 6 is a block schematic diagram of a digital valve in an automaticfluid control system employing a digital computer to determine the flowrate; and

FIG. 7 is a block schematic diagram of a digital valve in an automaticanalog fluid control system.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In FIGS. 1A and 1B, a fluid control system is shown which is constructedfrom standard pipes and plates. A section 1 of pipe is connected inseries with the conduits of a fluid system whose flow rate is to becontrolled. A section 2 of pipe having a smaller diameter than section 1is attached to section 1 at one end by a plurality of fins distributedevenly about the perimeter of section 2 such as those designated 3 and4. At the other end, section 2 is attached to section 1 by an annularterminating plate 5. Sections 1 and 2 are coaxially arranged about anaxis 8. A flow directing cone 6 has a base with a diameter equal to thatof section 2. The base of cone 6 is attached to the end of section 2where the fins are located and the apex of cone 6 extends away from theend of section 2. A truncated cone 7 is connected between the end ofsection 2 where terminating plate 5 is located and section 1. The baseof cone 7 extends away from the end of section 2. A plurality ofindividually digital valve elements 9 are disposed in a plane transverseto axis 8 for radial fluid flow from section 1 to section 2. A pluralityof individually digital valve elements 10 are also disposed in a planetransverse to axis 8 for radial fluid flow from section 1 to section 2.As used in this specification, the term `digital valve element` isdefined as a bistable valve element assuming only one of two states,that is, open or closed. Valve elements 10 are axially displaced fromvalve elements 9. Valve elements 9 are equidistantly spaced from axis 8and valve elements 10 are equidistantly spaced from axis 8.

The detailed construction of valve elements 9 and 10 is discussed belowin connection with FIGS. 5A and 5B. Briefly, each valve element has ahousing 20 that passes through aligned openings in sections 1 and 2. Aflow determining orifice 21 is located within housing 20 at the openingin section 2. The portion of housing 20 located between sections 1 and 2has a plurality of perforations 22 through it. A plug 23 is moved withinhousing 20 into one of two stable positions by a rod 24. In oneposition, plug 23 seals orifice 21 so the valve element is closed. Inthe other position, plug 23 is spaced from orifice 21 and out of theregion of housing 20 in which perforations 22 are located. The actuatingmechanisms for rod 24 are not shown in FIGS. 1A and 1B.

Transducers 25, 26, 27, and 28 sense the dynamic fluid conditions atvarious points in the fluid control system.

Fluid under pressure flows through the system generally along axis 8 inthe direction indicated by the arrows in FIG. 1B. The interior walls ofsection 1, the exterior walls of cone 6, the exterior walls of section2, and terminating plate 5 define an upstream chamber that isinterconnected by valve elements 9 and 10 to a downstream chamberdefined by the interior walls of cone 6, section 2, and cone 7. As thefluid enters the upstream chamber, the portion flowing in the vicinityof axis 8 is directed outwardly by cone 6. The fins are oriented tomaintain the direction of fluid flow substantially parallel to axis 8and prevent the formation of local disturbances as the fluid flow intothe annular portion of the upstream chamber in which valve element 9 and10 are located. Valve elements 9 and 10 are selectively opened tocontrol the rate of fluid flow from the upstream chamber to thedownstream chamber. Terminating plate 5 is spaced sufficiently far fromvalve elements 10 to permit approximately uniform fluid flow through allof perforations 22 in the open valve elements. Breaker plates 29 and 30are transversely situated in section 2 between the valve elements andcone 7. The fluid passing through the open ones of valve elements 9converges at one point of axis 8 and the fluid passing through the openones of valve elements 10 converges at another point on axis 8. The venacontracta which would otherwise form in section 2 at high pressure andflow rates is dissipated by the collision of the fluid streams passingthrough the open valve elements. Breaker plates 29 and 30 stabilize thestream of fluid as it leaves the downstream chamber to flow through theremainder of the fluid system.

The areas of orifices 21 of the valve elements are weighted such thatthe smaller areas follow a geometric progression of two and the largerareas deviate from a geometric progression. Assuming, for example, thatthe areas of orifices 21 of the twelve valve elements shown in FIGS. 1Aand 1B are weighted 1, 2, 4, 8, 16, 32, 32, 32, 32, 32, 32, and 32, thelargest flow rate attributable to any one valve element is 12 1/2percent of the maximum flow rate through the fluid control system.Accordingly, less transient disturbance results from a change in stateof valve elements and due to possible differences in their opening andclosing times than results in a digital fluid control system in whichall the orifice areas are weighted to follow a geometric progression oftwo. Further, the design of the valve elements is facilitated because nosingle valve element is required to accommodate onehalf the maximumfluid flow rate.

Ideally, all the valve elements of a fluid control system constructedaccording to the principles of the invention would lie in the same planetransverse to the direction of fluid flow in equidistant relationshipfrom a point lying on axis 8. In such case, the pressure drop acrosseach valve element is the same, with the results that the rate of fluidflow through the various valve elements is not interdependent. Thedissipation of the vena contracta and establishment of the same pressuredrop across each valve element cause the effective orifice area of allthe valve elements to be solely dependent upon the actual orifice area.This permits precise determination of the flow rate of a fluid flowcontrol system through the design of the individual valve elements.

In practice, the diameter of the pipes of a fluid flow control system issometimes too small to accommodate all the valve elements in a singleplane. In such case, the valve elements are arranged in two planes asillustrated in FIG. 1B. The valve elements with the smaller orificeareas are arranged in the plane furthest upstream, i.e., valve elements9 would have the smaller orifice areas. Thus, the effect of the statesof valve elements 9 on the fluid flow through valve elements 10 would beminimized and would, in most cases, be negligible.

In FIG. 2 an alternative embodiment of a fluid control systemincorporating the principles of the invention is shown. A bell reducer40 is connected to one side of an annular valve element block 41 and apipe 42 is connected to the other side of block 41. Block 41 has aplurality of radial bores 43 and a like number of axial bores 44 thatinterconnect the interior of bell reducer 40 with radial bores 43. Thebase of a flow directing cone 45 is directly attached to block 41 andits apex extends away from block 41 along the interior of bell reducer40. Fins such as that designated 46, which are distributed evenlybetween bell reducer 40 and cone 45, serve to support cone 45 andmaintain the direction of fluid flow. A breaker plate 47 is mounted inpipe 42 to stabilize the fluid stream after passage through the valveelements. Each radial bore has a flow determining orifice 48 at theentrance to pipe 42 and contains a digital valve element 49. Each ofvalve elements 49 comprises a plug 50 connected by a rod 51 to anactuator 52. Actuator 52 drives plug 50 into one of two stablepositions. In one position, plug 50 seals orifice 48, and in the otherposition, plug 50 unblocks orifice 48. The direction of fluid flow isindicated by the arrows in FIG. 2. The interior wall of bell reducer 40and the exterior wall of cone 45 define an upstream chamber while theinterior wall of cone 45 and the interior wall of pipe 42 define adownstream chamber. As in the embodiment disclosed in FIGS. 1A and 1B,the valve elements are radially disposed in a plane perpendicular to thedirection of fluid flow in the upstream and downstream chambers inequidistant relationship from an axis 53. Consequently, the pressuredrops across valve elements 49 are equal. Valve elements 49 are alsodisposed so the fluid passing through them into the downstream chamberconverges and dissipates the vena contracta.

In FIG. 3, a modified version of the fluid flow control system of FIG. 2is shown. The same reference numerals are used to identify thecorresponding parts in FIGS. 2 and 3. The modifications involved in thearrangement of FIG. 3 are as follows:

Bores 43 extend through block 41 at an acute angle to axis 53 instead ofperpendicular thereto; flow determining orifices 48 are located at theexit of bell reducer 40 instead of the entrance to pipe 42; and bores 44interconnect bores 43 with pipe 42. This arrangement provides a morecompact package than the arrangement of FIG. 2 because the lengthdimension of valve elements 49 is disposed in a position that has anaxial component as well as a radial component. It does not, however,produce as effective a dissipation of the vena contracta as thearrangements of FIGS. 1A and 1B and FIG. 2 because the convergence ofthe fluid streams from the valve elements has an axial component insteadof being purely radial.

In FIG. 4, an arrangement is shown that carries the modification of thearrangement of FIG. 3 to its limit. An upstream chamber is defined bythe interior wall of a housing 60 and the exterior wall of a flowdirecting cone 61. The walls of an axial bore 62 define a downstreamchamber. A plurality of axial bores 63 are formed in housing 60 aroundbore 62. Each of bores 63 has a flow determining orifice 64 at the exitof the upstream chamber. Radial bores 65 interconnect bores 63 with thedownstream chamber. Valve elements 66 are contained within bores 63.Each valve element has a plug 67 connected by a rod 68 to an actuator69. Actuator 69 drives plug 67 into one of two stable positions. In oneposition, plug 67 seals orifice 64, and in the other position, plug 67unblocks orifice 64. Breaker plates 70 and 71 are situated in thedownstream chamber. This arrangement of parts permits a compact packagewithout sacrificing efficiency in the dissipation of the vena contractaas in the arrangement of FIG. 3.

In FIGS. 5A and 5B, one of the valve elements of the fluid flow controlsystem of FIGS. 1A and 1B is shown in detail. With several minormodifications, this same valve element could also be used to advantagewith the arrangements of FIGS. 2, 3, and 4. The same reference numeralsare employed in FIGS. 5A and 5B and in FIGS. 1A and 1B to identifycorresponding parts. A weld pad 80 (not depicted in FIGS. 1A and 1B) isattached to the outside surface of section 1 wherein the valve elementis located. A radial bore 81 is formed in weld pad 80 and section 1 anda radial bore 82 is formed in section 2. Bores 81 and 82 are in axialalignment. Portions of the interior surfaces of bores 81 and 82 arethreaded and corresponding portions of the exterior surface of valvehousing 20 are threaded. The threaded connections between housing 20 andbores 81 and 82 are designated at 83 and 84, respectively. To remove thevalve element, housing 20 need only be unscrewed. An O-ring 85, which iscompressed between a flange 86 and the surface of weld pad 80, and anO-ring 87, which is compressed between a flange at the end of bore 82and the end surface of housing 20, provide fluid-tight seals between theexterior of housing 20 and bores 81 and 82. Housing 20 has a cylindricalchannel through it. A flow orifice block 79 is fitted into a recess inthe interior wall of housing 20 at the exit of the valve element. Block79 has a precisely determined opening through it which comprises orifice21. Plug 23 is bistable, i.e., it can assume one of only two stablepositions. In one of the stable positions orifice 21 is closed, and inthe other stable position orifice 21 is open. Plug 23 is a hollowcylinder 75 with a cover 76 at one end to which rod 24 is attached. Rod24 extends through a fluid-tight gasket 89 at the end of housing 20 to aconventional solenoid valve actuator 90 attached to housing 20 bysupports 77 and 78. A tension spring 91 is fixed at one end to actuator90 and at the other end to a spring keeper 92. Thus, when actuator 90 isunenergized, spring 91 urges plug 23 upward into the one stable positionshown in FIG. 5A. When plug 23 is in this stable position, the valveelement is open. Thus, fluid flows from the annular region betweensections 1 and 2 through perforations 22 into the interior of housing 20and then through orifice 21 into section 2, as depicted by the arrows inFIG. 5A. While the valve element is in its open stable state, theportion of the channel through housing 20 surrounded by perforations 22is in the dynamic fluid region of the system. The remainder of thechannel through housing 20 is in the static fluid region. Plug 23 liescompletely outside of the fluid stream in this static fluid region whenthe valve element is open. Therefore, variations in the position of plug23 in the open state that develop in the course of prolonged use do notaffect the flow rate through the open valve element.

Cover 76 has perforations 93 through it to balance the pressure on bothsides of plug 23. An annular plate 94 is fixed to cover 76. A sealingring 95 that surrounds plug 23 is attached to plate 94 by acompressible, fluid-tight metallic bellows 96. Bellows 96 functions as atension spring along the axis of cylinder 75. A ring guide 97 made of alow-friction material such as Teflon is embedded in an annular groove insealing ring 95 with a back-up spring 98. Spring 98 urges ring guide 97radially outward into contact with the interior walls of housing 20,thereby centering plug 23 as it moves within housing 20. When actuator90 is energized to close the valve element, plug 23 is driven downwardinto its other stable position until the bottom end surface of cylinder75 is seated on the top surface of orifice block 21 to form a seal. Inthis stable plug position, sealing ring 95 is seated on a steppedsurface 99 in the interior of housing 20 to form another seal. Surface99, which is perpendicular to the axis of cylinder 75, is disposedbetween the dynamic and static regions in the channel. The length ofbellows 96 is selected so it is somewhat compressed when sealing ring 95is seated on surface 99. Consequently, the restoring force exerted bybellows 96 on sealing ring 95 establishes a good, fluid-tight sealbetween surface 99 and sealing ring 95. As the parts of the valveelement wear in the course of prolonged use, the position of sealingring 95 is adjusted by bellows 96 to maintain a fluid-tight seal withsurface 99 in the closed state of the valve element. When the valveelement is closed, the fluid in the downstream chamber passes throughperforations 93 to balance the pressure on both sides of plug 23. Theseal between ring 95 and surface 99 prevents leakage however from theupstream chamber through perforations 22 and perforations 93 to thedownstream chamber and bellows 96 provides an elastic, fluid-tightconnection between ring 95 and annular plate 94 of plug 23.

In FIG. 6, a block diagram of an automatic control system is shown forthe digital valve arrangements described above. A digital computer 110produces binary signals that are coupled by line drivers 111 to theactuators of the elements of a digital valve 112. As the elements ofvalve 112 are opened and closed responsive to the respective binarysignals, the dynamic fluid conditions represented by block 113 vary. Asensor 114 detects the change in the dynamic fluid conditions. Theoutput of sensor 114 is converted to a digital signal by ananalog-to-digital converter 115 and applied to digital computer 110. Indigital computer 110, the actual dynamic fluid conditions are comparedwith the desired conditions stored therein and calculations are made togenerate binary signals for valve 112 so the actual dynamic fluidconditions approach the desired conditions. If the larger orifice areasof the elements of valve 112 deviate from a geometric progression asdiscussed above, the binary signals generated by digital computer 110would have to be modified prior to application to line drivers 111.

In FIG. 7, a block diagram of another system is shown for automaticallycontrolling the digital valve arrangements described above. A commandvoltage is applied to one input of a difference amplifier 120. Anintegrating operational amplifier 121 couples difference amplifier 120to an analog-to-digital converter 122. An encoder 123 couplesanalog-to-digital converter 122 to a bank of flip-flops 124. Linedrivers 125 connect the outputs of flip-flops 124 to the elements of adigital valve 126. As the elements of valve 126 open and close, thedynamic fluid conditions represented by block 127 change. The conditionsare detected by a sensor 128 that produces an analog signalrepresentative thereof at its output. The output of sensor 128 iscoupled through an amplifier 129 to the other input of differenceamplifier 120. Therefore, the output of difference amplifier 120 is thedifference between the actual and desired dynamic fluid conditions,i.e., the error signal. This error signal is integrated by operationalamplifier 121. Valve 126 is automatically adjusted to reduce theintegrated error signal.

The term "orifice" as used in this specification in connection with thevalve elements also includes a cavitation-venturi, in which the plugblocks and unblocks the valve elements at the venturi. In such case, thevalve elements are designed so the static pressure of the fluid at theventuri is the vapor pressure of the fluid plus system back pressure.Thus, the pressure drop across each valve element is the same, namelythe upstream pressure minus the vapor pressure, plus system backpressure.

What is claimed is:
 1. An automatic fluid flow control systemcomprising:a cylindrical upstream conduit adapted for fluid flow in agiven direction; a cylindrical downstream conduit adapted for fluid flowin the given direction, the downstream conduit being coaxial with andsmaller in cross-sectional area than the upstream conduit; a pluralityof individually actuatable digital valve elements interconnecting theupstream conduit and the downstream conduit, the valve elements beingdisposed between the upstream and downstream conduits in a planesubstantially transverse to the given direction such that fluidconverges radially through the valve elements transverse to the givendirection so as to form two substantially 90° bends in the flow pathfrom the upstream chamber to the downstream chamber, each valve elementhaving a flow determining orifice, a plug, and means responsive to abinary signal for controlling the movement of the plug so it assumes oneof only two stable positions corresponding to the values of the binarysignal, the plug closing the orifice in the one stable position andlying outside the fluid stream in the other stable position; means forgenerating a signal representative of the dynamic fluid conditions ofthe system; means for producing a plurality of binary signals equal tothe number of valve elements, the binary signals representing the valvestates required to make the dynamic fluid conditions satisfy apredetermined criterion; means for coupling to the binary signalproducing means the signal representative of the dynamic fluidconditions; and means for coupling the binary signals to the respectivevalve elements to place their plugs in the positions corresponding tothe values of the binary signals.
 2. The fluid flow control system ofclaim 1, in which the areas of the orifices are weighted such that thesmaller areas follow a geometric progression and the larger areas areequal.
 3. An automatic fluid control system comprising:an upstreamconduit; a downstream conduit; a plurality of individually actuatablebistable digital valve elements interconnecting the upstream conduit andthe downstream conduit, each valve element responsive to a binary signalassuming only one of two states corresponding to the values of thebinary signal, that is, open or closed, the valve elements beingdirected toward a point in the downstream conduit so the fluid flowingthrough the valve elements converges in the downstream conduit todissipate the vena contracta; means for generating a signalrepresentative of the dynamic fluid conditions of the system; means forproducing a plurality of binary signals equal to the number of valveelements, the binary signals representing the valve states required tomake the dynamic fluid conditions satisfy a predetermined criterion;means for coupling to the binary signal producing means the signalrepresentative of the dynamic fluid conditions; and means for couplingthe binary signals to the respective valve elements to place them in thestates corresponding to the values of the binary signals.
 4. The fluidflow control system of claim 3 in which each valve element has a flowdetermining orifice, a plug, and means for maintaining the plug in oneof only two stable positions, the plug closing the orifice in the onestable position and lying outside the fluid stream in the other stableposition.
 5. The fluid flow control system of claim 3 in which the valveelements have orifice areas that are weighted such that the smallerorifice areas follow a geometric progression and the larger orificeareas deviate from a geometric progression.
 6. A fluid flow controlsystem comprising:an upstream chamber; a downstream chamber; a pluralityof individually actuatable bistable digital valve elementsinterconnecting the upstream chamber and the downstream chamber, eachvalve element having a stationary valve housing disposed in the upstreamchamber, a plurality of perforations in the side of the housing toconnect the interior of the housing with the upstream chamber, anopening in the end of the housing to connect the interior of the housingwith the downstream chamber and a movable plug capable of assuming onlyone of two stable positions, the plug preventing flow from the upstreamto the downstream chamber through the perforations and the end openingin the one stable position and enabling flow from the upstream to thedownstream chamber through the perforations and the end opening in theother stable position, the plug lying outside the fluid stream in theother stable position; means for generating a signal representative ofthe dynamic fluid conditions of the system; means responsive to thesignal representative of the dynamic fluid conditions for producing aplurality of binary signals equal to the number of valve elements, thebinary signals representing the valve states required to make thedynamic fluid conditions satisfy a predetermined criterion; and meansfor coupling the binary signals to the respective valve elements toplace their plugs in the positions corresponding to the values of thebinary signals.
 7. The fluid flow control system of claim 6, in whicheach plug is disposed within its housing.
 8. The fluid flow controlsystem of claim 6, in which the flow rates through the respective valveelements in the other stable plug position are weighted such that thesmaller areas follow a geometric progression and the larger areasdeviate from a geometric progression.
 9. The fluid flow control systemof claim 6, in which the upstream and downstream chambers are adaptedfor fluid flow in a given direction and the valve elements are disposedin a plane transverse to the given direction such that the pressure dropacross each valve element is substantially the same.
 10. The fluid flowcontrol system of claim 6, in which the valve housing of each valveelement is cylindrical and the perforations are arranged in oppositelydisposed pairs in the side of the housing.
 11. The fluid flow controlsystem of claim 6, in which the upstream conduit is cylindrical andadapted for fluid flow in the given direction, the downstream conduit iscylindrical and adapted for fluid flow in the given direction, thedownstream conduit being coaxial with and smaller in cross-sectionalarea than the upstream conduit, the stationary valve housings beingcylindrical, having cylindrical axes, and being disposed so theircylindrical axes lie in a plane essentially transverse to the givendirection such that fluid converges radially through the orifices. 12.The fluid flow control system of claim 11, in which the valve housingseach have a threaded connection with the wall of the downstream conduitand are disposed in the space between the upstream and downstreamconduits.
 13. The fluid flow control system of claim 12, in which theplug has an actuator body, the actuator body having a threadedconnection with the wall of the upstream conduit and extending externalof the upstream conduit.
 14. The fluid flow control system of claim 13,in which the actuator body is integral with the housing and the plug isdisposed within the housing.
 15. A fluid flow control systemcomprising:an upstream chamber; a downstream chamber; a plurality ofindividually actuatable bistable digital valve elements interconnectingthe upstream and downstream chambers, the valve elements assuming onlyone of two states, open or closed, the orifice areas of the valveelements being weighted such that the smaller orifice areas follow ageometric progression and the larger orifice areas are equal to eachother; means for generating a signal representative of the dynamic fluidconditions of the system; means responsive to the signal representativeof the dynamic fluid conditions for producing a plurality of binarysignals equal to the number of valve elements, the binary signalsrepresenting the valve states required to make the dynamic fluidconditions satisfy a predetermined criterion; and means for coupling thebinary signals to the respective valve elements to place their plugs inthe positions corresponding to the values of the binary signals.